1. Field of the Invention
The invention relates to a brake circuit with at least one actuator operated by feeding of brake pressure and including a mechanical brake-pressure control device and an electrical brake-pressure control device.
2. Brief Description of the Background of the Invention Including Prior Art
Such a brake circuit is known from FIG. 2 of the European Patent Application EP-A-0,088,911. The electrical brake-pressure control device according to the reference EP-A-0,088,911 comprises the electrical part of a brake-value transmitter actuatable by a driver, an electronics with an actual value sensor, and the electrically controlled part of a relay valve controlled by two circuits. Upon actuation of the brake-value transmitter, the brake-value transmitter releases an electrical signal, and after processing in the electronics, the electrical signal is fed to the electrical part of the relay valve. The electrical signal is transformed by the electrical part of the relay valve into a brake pressure for the actuator. The mechanical brake-pressure control device comprises a pressure part of the brake-value transmitter and the pressure-dependent part of the relay valve controlled by two circuits. "Mechanical" is a designation of this brake-pressure control device because the actuating value fed into the brake-value transmitter, is transformed in a conventional way with mechanical means into a brake pressure for the actuator.
The conventional brake circuit is actuated preferably by the electrical brake-pressure control device. This occurs in that the electric signal initiates a quicker actuation of the relay valve controlled by two circuits, because of the large propagation speed of the electrical signal in the corresponding lines, than is the case of the brake pressure transmitted by the pressure part of the brake-value transmitter. In case of a fault in the electrical brake-pressure control device, the brake circuit remains functional based on the mechanical brake-pressure control device, even though this function is at a lower speed.
It is disadvantageous in connection with this conventional brake circuit that the brake circuit is not accessible to a load-dependent automatic brake-force control with the electrical brake-pressure control device. In a conventional way, the electrical signal, delivered by the brake-value transmitter, and thus the brake pressure for the actuator, are corrected dependent on load for achieving such an automatic brake-force control. In such a case, the electrical brake-pressure control device would in fact effect a quick response of the relay valve and thus of the brake circuit according to the conventional circuit. Following to the access and activation phase, however, the brake pressure, transmitted by the pressure part of the brake-value transmitter and not load-dependently corrected, would assume predominance during the control of the relay valve and the load-dependent correction of the brake pressure, based on the electrical brake-pressure control device, would become ineffective.